This invention relates to image projection systems and more specifically to light-collimating lenses and associated image-enhancing light filters for displaying projected images over large-area display screens.
Certain contemporary large-screen video displays commonly rely upon image projection onto the rear surface of a display screen. Several forms of rear-projection screens have emerged ranging from slightly translucent diffusers to more complex multi-layer optical filters that modify viewing angles, image contrast, and the like, attributable to particular layered structures of such filters. Image-enhancing filters of this type are described in the literature. (See, for example, U.S. Pat. No. 6,076,933 and U.S. Pat. No. 5,781,344.)
One common requirement of image-enhancing viewing screens in such applications is the need for substantially collimated light of the projected image incident upon the rear surface of the viewing screen. This may be accomplished approximately, but not commercially practically, in projection systems that project an image over a great distance. More commonly and practically, Fresnel lenses are introduced into an image projection system to optically reform the path of image light from a projected cone of diverging light rays into a substantially collimated pattern of parallel light rays incident upon the rear surface of the viewing screen. Of course, the path of projected image light may be optically folded and otherwise manipulated using reflectors and supplementary lenses within associated cabinetry in order to provide substantially collimated incident light images to the rear surface of a viewing screen.
Light-image projection systems have selected physical characteristics including a certain projection distance and a certain diagonal dimension from which an xe2x80x9cf xe2x80x9d number, or f/#, may be determined as the ratio of projection distance (from the projector to the viewing screen) to the diagonal dimension of the focused projected image. The practical necessity of shortened projection distances for convenient enclosure within modest-size cabinetry is exacerbated by popular demands for larger viewing screen that now result in f/#""s for projection systems which are commonly less than 1.
Additionally, a Fresnel lens has a maximum diagonal dimension of its active lens area, and has a selected focal distance from which an f/# for the lens can be similarly calculated as the ratio of the focal distance to the diagonal dimensions of the lens area Ideally, the f/# of the projection system should substantially match the f/# of the Fresnel lens to assure that the projected image light emerges from the Fresnel lens as substantially collimated rays incident on the rear surface of the viewing screen. However, such matching of f/#""s over a range of diagonal screen sizes and projection lengths within acceptable cabinet sizes commonly requires numerous Fresnel lenses of different sizes and designs for requisite matching of the applicable f/#""s. And, since tooling for forming Fresnel lenses is complex and expensive, it is desirable to minimize the required variations in sizes and lens designs commonly needed to match wide ranges of projection systems.
In accordance with the present invention, mismatching of projection and lens f/#""s within tolerable limits are resolved with negligible visible degradation of the projected image viewed through an image-enhancing filter. Specifically, a multi-layered image-enhancing filter including layers of transmissive beads and masking and index-matching materials, for example, as described in the aforementioned patents, can tolerate significant amounts of off-axis image (i.e., non-collimated) light with only negligible reduction in image quality. Accordingly, tolerable mismatching of projection distance and focal length of the Fresnel lens is utilized in combining a few Fresnel lens designs with a wide range of projection systems. In addition, the overall range of viewing screen sizes up to approximately 5 feet diagonally is divided into sub-ranges that require Fresnel lenses which are each designed around parameters at the mid-range of each such sub-range. This reduces the number of different lens designs required to substantially match the f/#""s of Fresnel lenses with the f/#""s of the projection systems required to produce images variously of about 40xe2x80x3 to 70xe2x80x3 diagonal dimensions on viewing screens in conventional height-width aspect ratios of 3:4 and 9:16.